This invention relates to a method for dewatering or drying organic solid substrates which contain a large amount of water. The method has specific relevance with respect to fossilized biomass materials such as peat and coal but also has application with respect to various other biomass substrates such as agricultural and forestry residues (bagasse, corn stover, saw dust, etc.), crops deliberately grown for their fuel or chemical value, and sewage sludge. The present invention provides a heat-efficient method for removing water from such materials prior to the conversion of the solid to a more valuable product.
Peat and certain other organic solid substrates contain substantial amounts of water. It is known that peat can contain up to 90 percent or more water, by weight, and this has been the major deterrent to its wider use as a fuel source. It is desired to use peat and some other biomass substrates as fuel sources by direct combustion or by high temperature conversion to a more convenient fuel such as a fuel gas. As much of the water as possible must be removed from these materials prior to conversion since much of their potential heating value would otherwise be consumed in the evaporation of the water contained therein. For example, when peat is burned directly, much of the heat generated must be used to evaporate the water originally present in the peat. As a result, the temperature produced by combustion is low and the efficiency of combustion heat transfer is greatly reduced. These disadvantages cannot be overcome by merely heating the peat to evaporate the water prior to combustion because the same heat and cost inefficiencies would result, unless, of course, the heat was free such as "field drying". However, this leads to other problems such as containing the dried material when it is dispersed widely over the landscape.
U.S. Pat. No. 3,327,402, issued June 27, 1967, describes a process for drying coal fines at low temperatures by contacting them with a volatile low molecular weight solvent such as methanol in which water is highly soluble at low temperatures to remove the water from the coal fines. The coal fines are heated to remove the volatile solvent and the solvent is recovered from the water-solvent mixture by distillation. U.S. Pat. No. 4,014,104, issued Mar. 29, 1977, discloses a method of drying a moist particulate carbonaceous material, which may be peat, by adding methanol to the material followed by application of heat to remove both the solvent and water from the materal. Again, the solvent is separated from the water by distillation. Neither of these patents discloses the use of a secondary solvent to separate the primary solvent from the water.
U.S. Pat. No. 4,212,112, issued July 15, 1980, discloses a method for drying solid carbonaceous materials wherein they are mixed with benzene to form a constant boiling mixture of water and benzene which is not susceptible to composition change except by evaporating enough of the mixture so as to consume one constituent. The water and benzene are separated by heating the mixture until the benzene is completely vaporized. The condensed vapor forms two phases which are easily separable. U.S. Pat. No. 4,223,449, issued Sept. 23, 1980, discloses a heat-efficient method for dewatering solids such as peat whereby the solids are contacted with a liquid solvent such as benzene or toluene at a high temperature, the solids are removed, and the liquid stream is cooled to a lower temperature, less than 150.degree. F., thereby causing the water to separate from the solvent as a separate phase. Neither of these patents disclose the use of a secondary solvent to separate the primary solvent from the water which it removes from the carbonaceous material.
It is an object of this invention to provide an energy and cost-efficient method of removing water from organic solid substrates such as peat by means of a primary solvent which removes the water from the substrate and a secondary solvent which separates the primary solvent from the water. This eliminates the need for energy and cost inefficient methods of separating the primary solvent from the water such as distillation. The secondary solvent is chosen so that it can be easily and cheaply separated from the primary solvent.